421m
3
DOWNWARD EXPANDER RELEASE control
Front & Rear Panel Overview
Chapter 3
DOWNWARD EXPANDER THRESHOLD - Determines the level below which the downward
expander operates, which reduces the gain at a 1:2 dB ratio.
At the max CCW setting of the control, the threshold setting is automatic; determined by the
setting of the AUTO-RELEASE control. In this case, the downward expander acts more like a gate
(very high ratio) NOTE: the silence detector thinks that acoustic feedback is silence!
At the max CW setting, the threshold setting is low enough that all signals are above threshold,
therefore the expander is bypassed.
DOWNWARD EXPANDER RELEASE - Determines the rate of decay for the downward expander.
EXPAND LED - Tells when the downward expander is active.
LEVELER ACTIVE LED - Indicates the leveling activity. If the LED is off, then the gain of the AGC-
Leveler is frozen at the last setting. The signal level where the gain is frozen is determined by the
AUTO-RELEASE LEVEL control.
AUTO-RELEASE LEVEL - Determines the level that the input signal must exceed to allow the AGC-
Leveler to respond. If the signal level is below this threshold (active LED OFF), then the AGC gain
is frozen and the AGC allows the signal level to fall to zero. This keeps the AGC-Leveler from
undoing fades, etc.
Another way to understand this control is to think of it in terms of the minimum signal level
required to make the AGC-Leveler try to maintain the Target Output Level. Once the signal falls
below the AUTO-RELEASE setting, the AGC-Leveler allows the signal to decrease. When the
input signal exceeds the AUTO-RELEASE setting, the AGC-Leveler once again tries to maintain
constant output level.
RECOVERY RATE - Sets the basic recovery-time. Fast for speech, slow for music or music/speech.
Fast might work for music in special applications.
RATIO - Determines the input/output ratio. This control instructs the 421m how aggressively you
want to maintain the target output level. Using a 2:1 ratio, a 2dB drop in the input level results in a
1dB drop at the 421m’s output. With a 4:1 ratio, a 4dB drop at the input is limited to a 1dB decrease
at the output. The 421m computes the additional make-up gain needed for the ratio setting and
applies it to the VCA so that the output level doesn t change with the setting of the ratio control.
TARGET LEVEL - Sets the desired output level.
This chapter provides a basic overview of the 421m by describing the input and output connec-
tions, power connection, operating controls, and front panel LED indicators.
AGC-LEVELERDOWNWARD EXPANDER (NR)
421m
AGC-LEVELER
AUTO
RECOVERY
AUTO-RELEASE
BYPASS
LEVEL
RATE
2:1
INCREASE
DECREASE
FAST
SLOW
ACTIVEEXPAND
-45-20
BYPASS
TARGET OUTPUT LEVEL
HIGH
LOW
4:1
RATIO
SLOW
FAST
RELEASETHRESHOLD (Vu)
-35
AGC AUTO-RELEASE
control
RECOVERY RATE
button
RATIO control
TARGET
OUTPUT LEVEL
LEVELER
ACTIVE LED
DOWNWARD EXPANDER
THRESHOLD control
EXPAND LED
Front panel view (left)
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